Could it hurt to hyperventilate after being knocked out?

Big and vague ?: I'm not sure if you are asking about medically induced hyperventilation after head trauma to reduce intracranial pressure, or if you just had a knock to the head where you lost consciousness yourself. If it's the latter, get seen by a doctor. Even minor concussions have important medical implications and you need evaluation and follow up. Best of luck. Seriously, see a doc ASAP.